The radiation theory and structure of light-emitting diodes (LED) are different from that of conventional lighting sources, such as fluorescent lamps or incandescent lamps. LEDs have advantages as low power consumption, long life-time and fast responsive time. Moreover, LEDs are compact, shockproof, and environment-friendly, so LEDs are widely adopted in the market. For example, LEDs can be used in display apparatus, indoor or outdoor lighting, data storage devices, communication devices, medical devices, and so on.
An LED normally include a substrate, a light-emitting stack including an n-type semiconductor layer, an active layer, and a p-type semiconductor layer on the substrate for emitting light.
The total amount of light emitted from an LED device (i.e., the total integrated flux) is the integrated flux emitted from the topside of the device added to the integrated flux emitted from the sidewalls of the device. Side-emitted light is typically guided to the sidewalls of the device by a waveguide created by reflective surfaces formed of various layers having different refractive indices. Waveguided light typically undergoes several reflections along the light path to the sidewalls of the LED device, and therefore loss intensity by each reflection. It is advantageous to extract as much light as possible from the topside of the device by reducing internal losses and increase the total integrated flux. In GaN series LED, the refractive indices of sub-layers of the n-type semiconductor layer or the p-type semiconductor layer are about similar, the n-type semiconductor layer or the p-type semiconductor layer can be regarded as a single semiconductor layer for designing the distance between the active layer and the reflective layer in order to reduce the interference between the light from the active layer propagating towards the topside of the LED device and the light reflected by the reflective layer.